The need to detect electric power cables buried underground frequently arises when excavations are required and there is the risk of damage to underground cables. Although in normal circumstances the location of buried cables is generally recorded, during maintenance work, for example, cables can be moved and therefore prior records may not always be reliable.
Electric cables are also required for feeding power to equipment during underground construction. As a result of this underground activity, an energized electrical cable must be available during the process of work and owing to the nature of the work is apt to vary in location and very possibly also in length. Such a cable generates an alternating magnetic field with frequencies, typically around 50 Hz or 60 Hz, according to the domestic power supply network to which the cable is connected and this magnetic field allows remote detection of the cable.
For example, U.S. Pat. No. 6,356,082 (Alkire et al.) issued Mar. 12, 2002 discloses a radio link established between a portable receiver used by an operator to trace the underground portion of a utility and a transmitter which induces a magnetic signal in the utility. Circuitry in both transmitter and receiver allows the operator who is remote from the transmitter to interrogate the transmitter from the receiver for essential information and to control operating functions at the transmitter.
U.S. Pat. No. 5,006,806 (Osman et al.) issued Apr. 9, 1991 discloses methods and apparatus employing permanent magnets for marking, locating, tracing and identifying hidden objects such as buried fiber optic cables. The objects are provided with elongated permanent magnet identifier devices having magnetic fields that may be detected at a distance from the objects.
It is also known to employ airborne apparatus for cable detection. For example, WO 96/23235 (Mulder et al.) published Aug. 1, 1996 uses an antenna for inducing a signal from a power line magnetic field, pre-amplified, passed through a low pass filter and gain stage contained within a servo loop to remove DC offset, and amplified in two channels providing voltage gains of unity and 256. A dual channel, 22 bit sigma-delta converter digitizes the high and low gain signals and outputs directly to a digital signal processor, which selects between default high gain and low gain signals, high gain buffer overflow causing the switch. Samples are scaled, windowed and passed to a Fast Fourier Transform (FFT) module to yield solutions which represent the magnitude and phase of the signal, which are processed to discard frequencies outside the power line frequency +/−10 Hz. The remaining solutions are passed to an estimator, wherein the magnitude squared of each FFT bin is calculated and stored in an estimate buffer, which holds the last 8 such results. Signal estimation is provided by averaging the last n values for n=1 to 8 in the PSD estimate buffer, the estimates S(1) to S(8) being used in the wire decision processing. Noise estimation is made wherein the average PSD estimate is used to estimate the noise. The current threshold is updated when the S(8) value is below the current threshold minus a predetermined amount. If S(1) exceeds the current threshold multiplied by the margin for the current sensitivity setting, then a wire is considered to have been detected.
The apparatus described in WO 96/23235 can be applied to the detection of current-carrying underground cables but is suited only for the detection of an isolated electrical wire and not for the detection of current-carrying underground cables owing to possible presence of other buried cables in close proximity. In the case of detection of isolated electrical wires the problem can be presented as a general problem of detection of noise. In the case of detection of underground buried electrical cables the problem is much more complicated owing to the possible presence of other buried cables in close proximity. Consequently, the airborne monitoring of underground activities is characterized by a relatively high level of interference picked up by magnetic sensor from on-ground distributed domestic power networks. In this case the problem of signal-to-interference ratio is more significant than signal-to-noise ratio for efficient detection of buried cable. No solution to this problem appears to have been proposed in the art.